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Case study 5 - Main cranes structure excessive vibration and stress

On my offshore support vessel I had crane’s column lengthen. Since then increased deflections and vibrations during operation were noticed. After AHC system adjustment situation got better. I am still not sure if vibrations and strain levels are in safety limits? Can you advise me?

Analysing obtained information, Info Marine Service Engineer concluded that several of simultaneous both tensiometric and multichannel vibration tests should be undertaken. Design of the measurement system consists of 8 strain gauges and 14 vibration sensors placed at crucial places of crane structure.

Measurements were performed for static load, during normal operation with AHC mode on, lowering and lifting weights. 40 tons and 100 tons weights were used. Additional tensiometric measurements were carried out with AHC mode turning off. Since operation of AHC system has an effect on the tension in crane’s construction depending on sea state, measurements were carried out in two different weather conditions .

The most important places of measurements were located on main boom - measurement points MP1, MP2 and MP3 , where strain gauges were glued.

It was noticed that the most fatigue stress is observed during AHC mode turning off.

Measured stress in point MP1

Measured stress in point MP2

Measured stress in point MP3

Afterwards collected data was thoroughly analysed. Final conclusion is that measured stress and vibrations are allowed by DNV GL standards in the whole range of measured working conditions, hence it is safe to operate the crane.